Continuous preparation of organic isocyanates

Abstract

A two-stage continuous preparation process for organic isocyanates uses a primary amine which is reacted with stoichiometrically excess phosgene at a gauge pressure of 10 Kg/cm2 or lower and a temperature of 60 DEG -100 DEG C. in an inert organic solvent. Then, the resultant reaction mixture is maintained at a gauge pressure of 10 Kg/cm2or lower and a temperature of 120 DEG -160 DEG C. until the reaction is completed.

What is claimed is: 
     
       1. In a process for continuously preparing an organic isocyanate by reacting a corresponding organic primary amine and stoichiometrically excess phosgene in an inert organic solvent, the improvement which comprises the steps of: (a) bringing the organic primary amine in a practically dispersed state into contact with phosgene at a gauge pressure of 2-7 Kg/cm 2  and a temperature in the range of 60°-100° C., thereby forming a corresponding organic carbamyl chloride, converting the bi-produced hydrochloric acid salt of said amine, and phosgene into the organic carbamyl chloride and decomposing 30-70% of the thus-formed organic carbamyl chloride present in the reaction mixture into the organic isocyanate; and   (b) maintaining the resultant reaction mixture at a gauge pressure of 3-7 Kg/cm 2  and a temperature in the range of 120°-160° C., thereby converting the hydrochloric acid salt of said amine and phosgene present in the reaction mixture into the organic carbamyl chloride and completing the decomposition of said organic carbamyl chloride into said organic isocyanate.   
     
     
       2. The process according to claim 1, wherein said organic primary amine is practically dispersed by supplying through dispersing means said organic primary amine into phosgene in an amount of at least 1.5 moles per each amino group of every mole of said amine, which phosgene is recirculated together with the inert organic solvent through a tubular recirculating line. 
     
     
       3. The process according to claim 1, wherein said organic primary amine is practically dispersed by supplying through dispersing means said organic primary amine at a flow rate of 10-30 m/sec into phosgene in an amount of at least 1.5 moles per each amino group of every mole of said amine, which phosgene is present together with the inert organic solvent in a tank-shaped reaction vessel. 
     
     
       4. The process according to claim 1, wherein the reaction under pressure in step (b) is carried out in a recirculating line which includes a heating tube and a gas-liquid separator. 
     
     
       5. The process according to claim 1, wherein said organic primary amine is selected from tolylene diamine and diaminodiphenylmethane. 
     
     
       6. The process according to claim 5, wherein phosgene is supplied in step (a) in an amount of 3.4-8 moles per mole of the amine. 
     
     
       7. The process according to claim 5, wherein the inert organic solvent is o-dichlorobenzene. 
     
     
       8. The process according to claim 1, wherein the residence time of the reaction mixture in step (a) is 30-120 minutes. 
     
     
       9. The process according to claim 1, wherein the residence time of the reaction mixture in step (b) is 10-120 minutes. 
     
     
       10. The process according to claim 1, wherein the concentration of the organic primary amine in step (a) is 10-30% by weight in a dispersion zone of said amine. 
     
     
       11. The process according to claim 1, wherein the concentration of the organic isocyanate at the end of the reaction in step (a) is 10-25% by weight. 
     
     
       12. The process according to claim 5, wherein the reaction temperature in step (a) is 70°-90° C.